The present invention relates to a pressure transducer for obtaining an electric signal corresponding to a pressure through the use of variable capacitance elements whose impedances vary with a displacement corresponding to the pressure.
A known capacitance type differential pressure transmitter is, for example, disclosed in Japanese Published Patent No. 23916/74. In a pressure sensor described therein, a cell is divided into two metal parts, a metallic film is formed on each metal part through an insulator and a sensing diaphragm forming a condenser plate is held by the two metal parts together with the metallic films. By pressures applied to pressure receiving diaphragms provided on both sides of the pressure sensor body, the sensing diaphragm is displaced to cause changes in the capacitances of first and second variable capacitance elements formed between the sensing diaphragm and the metallic films. From these capacitance variations are obtained an electric signal corresponding to the pressure.
A prior art example of a converter circuit for use with such a pressure is, for instance, described in Japanese Published Utility Model No. 21643/72. The converter circuit proposed therein employs an oscillator for generating an AC signal whose amplitude is controlled by a control unit and the AC signal is applied to the first and second variable capacitance elements of such a pressure sensor. An output voltage proportional to a difference between currents flowing through the variable capacitance elements is obtained by two rectifier elements connected to respective ones of the variable capacitance elements with reverse polarities and a smoothing circuit. At the same time, a voltage proportional to the sum of the currents is obtained and compared with a reference voltage. The resultant difference voltage is fed back to the control unit. By properly selecting the values of circuit elements, there is obtained a signal corresponding to the ratio between the difference and sum of the first and second variable capacitance elements.
The above-mentioned conventional capacitance type pressure sensor employs a flat diaphragm as the sensing diaphragm and holds it with the two metal parts therebetween. However, the pressure sensor is liable to be thermally distorted due to a difference between the expansion coefficients of the sensor body and the sensing diaphragm. Additionally, it is subjected to a mechanical distortion when the entire pressure sensor is clamped by fitting flanges. Moreover, the sensor body is composed of two separate metal parts which must be welded together with the sensing diaphragm held therebetween which makes the manufacturing cost high.
The prior art conventional converter circuit has the defect that its output varies with ambient temperature variations. That is, the rectifier elements used in the converter circuit may have different temperature characteristics and the output signal is affected by such characteristic differences of the rectifier elements. Further, in the above conventional converter circuit, currents flowing in the variable capacitance elements must be controlled to be a constant value which makes the circuit arrangement complex.
Accordingly, it is an object of the present invention to provide a pressure transducer which has a pressure sensor free from the above-mentioned defects of the prior art, simple in structure and which is not susceptible to influences of thermal and mechanical distortions.
Another object of the present invention is to provide a pressure transducer which has a converter circuit capable of stably amplifying an electric signal detected by the pressure sensor and transmitting the amplified signal to a receiving instrument.
Yet another object of the present invention is to provide a pressure transducer which has a pressure sensor in which variable capacitance elements are simply mounted and their electrodes are led out of the sensor body while being electrically insulated therefrom.